This invention relates broadly to the production of three-dimensional structural components that make up the structural cores of lightweight composite sandwich panels and other load-bearing structures that have a high level of strength relative to structure weight. More specifically, the invention discloses a method and apparatus for forming from a fiber slurry a lightweight, fiber truss having three-dimensional features that reinforce the truss and that reinforce composite structures containing the truss.
A three-dimensional fiber truss is defined herein as a fibrous element that has a strategically engineered three-dimensional shape designed to create, either stand-alone or in combination with other elements, a structural framework for load-bearing purposes. Typically, the wall thickness of a three-dimensional fiber truss will be small compared to the overall height of the three-dimensional features making up the fiber truss. With relatively thin walls, fiber mass is minimized relative to the strength and rigidity that may be attained in products that incorporate the fiber truss.
In many panel applications, the three-dimensional structure of the fiber truss consists of a series of hollow protrusions from a truss base. In this form, the fiber truss is bonded to exterior skins or to other fiber trusses. The composite structure formed in this way completes the three-dimensional truss work of the fiber truss element, and produces a product having a high level of strength and rigidity. A composite structure in which the fiber truss functions as a structural core between exterior skins is commonly known as a xe2x80x9csandwich panel.xe2x80x9d Because the flat sheets that form the exterior skins of the panels carry most of the stresses during bending, these composite sandwich panels are sometimes referred to as xe2x80x9cstressed-skin panelsxe2x80x9d.
Fiber trusses can be manufactured using a wide variety of methods and a diverse selection of materials, including metal filaments, wound fiber composites, folded paper sheets, and wood fiber compositions. The present invention is practiced using a narrowed class of materials composed of fibers that may be suspended and randomly distributed in a carrier fluid. Fibers of this type may be derived from various lignocellulosic materials as well as a wide range of synthetic materials. Different fiber types may be mixed together in the carrier fluid and/or a number of chemical additives may be applied to the fiber or fiber blend to achieve specific properties. The fiber trusses described in U.S. Pat. Nos. 5,900,304; 4,495,237; and 4,348,442 are examples of fiber trusses that may be produced using the disclosed method and apparatus.
When the fiber truss or core element is sandwiched between skins to complete the three-dimensional truss work, the resulting composite sandwich panel behaves in a manner analogous to a common I-beam, which is a very efficient engineering structure. By analogy, the fiber truss or core element corresponds to the central rib of the I-beam and the skins correspond to the top and bottom flanges of the I-beam. In bending an I-beam parallel to the central rib, one flange of the I-beam undergoes compression, while the other flange is in tension. In an analogous manner, bending of a composite panel places one of the skins in compression while the other skin is held in tension.
Numerous applications for sandwich panels exist in construction, furniture, material handling and packaging industries. The most common sandwich panel is the corrugated panel used extensively in light-weight box construction. In this case, the structural core is formed by bending thin paper sheets into an undulating or corrugated pattern. The corrugated cores are glued between paper skins to form the familiar corrugated panels. Numerous other lightweight structural cores and sandwich panels have been disclosed in the prior art, but only a few of these have been successfully commercialized. The honeycomb core is one of the more successful of these other cores.
The relative success of the honeycomb core and the corrugated core derive from the relative simplicity of their structural shapes and formation methods. There has been a need to define a correspondingly simple and cost-competitive method for constructing more complex structural core shapes which have improved structural properties. The present invention focuses upon such a method for the manufacture of a wide range of lightweight structural cores, and other load-bearing structures, using both natural plant fibers and synthetic or man-made fibers.
In the initial step of the method, the fibers are mixed in a carrier fluid to form a random or quasi-random fiber distribution. The mixture comprising fiber and carrier fluid will be referred to as a xe2x80x9cfiber slurryxe2x80x9d in this disclosure. Slurry formation is followed by a step in which the carrier fluid is almost completely discharged from the slurry and the retained fiber is concentrated and compacted in a wet-pressing operation, forming a pre-form fiber truss. Finally, the pre-form fiber truss is dried to form a finished fiber truss.
While the disclosed invention emphasizes the formation of fiber trusses used to construct core elements used in composite panels, the invention also can be used to form a wide range of molded fibrous products and components, including egg cartons, produce trays, conformal packaging, molded packaging inserts, and molded pallets. Well known conventional fiber-molding processes used to make these types of products begin by extracting fiber from a slurry and depositing the fiber onto a single forming mold using vacuum suction through a single porous mold containing an overlay of screen elements. Occasionally, air pressure is applied to the slurry either alone or in combination with vacuum extraction to increase the pressure differential which drives the carrier fluid through the porous mold and screen elements.
The equipment used for these conventional forming processes is fragile and expensive. For the more commonly utilized vacuum-only forming process, the pressure differential exerted across the porous mold and screen elements is limited to a maximum level equal to ambient air pressure. This low pressure differential results in comparatively slow carrier fluid removal and fiber formation. Fiber formation slows down substantially as product thickness increases.
In both vacuum-only processes and conventional gaseous-pressure-forming methods, slurry fluid-discharge and fiber deposition forces are applied across only a single forming surface. Single-surface fiber formation is not only a slow process, but fiber tends to be lumpy and wall thickness erratic. These conventional processes generally yield wet pre-forms having low solids content and high water content. High water content results in a fragile pre-form that is difficult to handle. The need to remove large quantities of water per pound of fiber leads to inefficiencies in energy usage and high costs in drying the wet pre-forms.
In the prior art, several improved methods are disclosed for forming from fiber slurries fiber trusses and other fiber structures, but these prior methods are still relatively complicated and expensive. For example, Setterholn and Hunt in U.S. Pat. No. 4,702,870 describe a method for forming specific three-dimensional structural components from wood fiber. Their method and apparatus require the use of a fragile resilient mold insert to form three-dimensional features in the finished fiber product. The use of resilient mold elements results in slow drying of the fiber, long production cycles, and maintenance problems associated with the fragility of the resilient mold elements. While the product formed according to the method of Setterholm and Hunt has excellent mechanical properties, the aforementioned problems associated with the resilient mold elements make the product expensive and limit the scope of applications. Nonetheless, the product has attracted considerable commercial interest and has had some degree of marketing success. Much greater success could be enjoyed with a simpler and more rapid manufacturing method.
A method and apparatus for producing a product similar to that of Setterholm and Hunt is disclosed by Emery in U.S. Pat. No. 5,833,805. In this case, solid mold elements are used, which allow rapid drying of the fiber form. The method and apparatus of Emery thereby improves the speed of production of three-dimensional fiber structures over that of Setterholm and Hunt. Nonetheless, the fiber is initially formed using a vacuum molding process that limits production speeds for the same reasons described previously for other vacuum-forming process. The method and apparatus disclosed by Emery also appears quite complicated. In addition, both the method of Setterholm and Hunt and that of Emery can be applied only to products having limited fiber truss thickness and a very specific structural form.
A method and apparatus for pulp molding have been disclosed by Shetka in U. S. Pat. Nos. 4,994,148 and 5,064,504. In this case, a pulp press and forming method are disclosed that utilize a pulp-molding chamber having interior walls comprised of rigid screens, through which water can pass, and rigid support plates outboard of the screens. The support plates contain fluid-discharge channels that collect water passing through the screens. The pulp is de-watered and compressed by squeezing the pulp between a moveable wall and a fixed opposed wall. Water is discharged through all interior surfaces of the pulp-molding chamber.
The use of screened walls on all interior surfaces is not conducive to the use of a sliding seal, captured in the periphery of the moveable wall, that slides along the surrounding screened walls. In fact, Shetka does not suggest the use of any seal in the space between the moveable wall and the surrounding screened wall. Without a seal in this space, leakage paths exist around the moveable wall, leading to the undesirable deposition of fiber in the space between the moveable wall and the wall which surrounds it. The leakage in this space also relieves the pressure that is applied to the pulp, leading to relatively low forming pressures. Low forming pressures result in low formation speeds.
It is apparent that the invention of Shetka is taught principally for the formation of relatively thick objects, such as blocks, flat boards, and solid panels. The fluid-discharge openings in the side walls adjacent to the moving wall of the device have advantages in the formation of thick objects, since substantial carrier fluid may be discharged from the perimeter of thick objects. Fluid-discharge openings in the side walls have no advantage in the formation of thin fiber trusses, since very little fluid discharge occurs from the thin edges of typical fiber trusses. Therefore, if Shetka had intended to include the formation of thin fiber trusses within the scope of his invention, he would have suggested the elimination of perforations in the walls surrounding the moveable wall. No such suggestion is made.
In fact, for thin-truss formation, there is a disadvantage in discharging the carrier fluid of a fiber slurry through the side walls. With side-wall fluid-discharge, fiber is deposited along the side wall and swept ahead of the moving wall during truss formation. This process results in excessive build-up of fiber around the perimeter of a thin truss at the expense of reduced fiber mass and fiber density over the rest of the truss, producing a poorly formed truss having weak interior regions.
Neither does Shetka suggest the formation of relatively complex three-dimensional surface features present on both sides of typical fiber trusses. Instead, Shetka suggests only the formation of shallow embossments on a single surface for decorative effect. Shetka does not suggest the use of these embossments for structural purposes. To form the embossments, Shetka inserts a solid, non-permeable template against the foraminous lower wall opposite the moveable wall of the invention. The template contains the impression of the desired embossment. Since the template covering the lower wall is impermeable, pulp fluid-discharge through the lower wall is blocked and the formation process is slowed considerably in forming the embossments.
Another device for forming three-dimensional molded objects from fiber slurries was disclosed by Posch et. al. in U.S. Pat. No. 3,832,108. The device includes a basic form, a moveable pressing form and a moveable frame surrounding the fixed basic form. The basic form and the pressing form are both liquid-pervious. The basic form is immersed in a fiber slurry and a layer of fiber is collected along the surface of the basic form using a vacuum forming process similar to that used in the conventional manufacture of molded fiber products. Once the fiber layer forms, the moveable pressing form compresses the deposited fiber layer, producing a smooth, accurately dimensioned surface. As was the case in common forming methods, fiber deposition occurs only over a single forming surface as carrier fluid is discharged through the forming surface under vacuum forces, leading to the same limitations in forming speed discussed earlier for other vacuum-forming processes.
Complete immersion of the basic form in a bath of fiber slurry, taught by Posch, et. al., necessitates withdrawal of the basic form from the slurry bath, or removal of the slurry from the basic form in order to recover the formed truss after it has been compacted by the pressing forms. In compaction and molding of the fiber layer with large compressive forces, bulky forms and a heavy mechanical support system are required. Movement of these very heavy components for the purpose of immersion in the fiber slurry followed by withdrawal of these components from the slurry, as implicit in one embodiment of the method of Posch et. al., would be complex, slow, and very costly.
The problem of devising a high-speed, cost-effective method for forming from a fiber slurry a wide range of three-dimensional fiber truss structures therefore remains extant.
It is an object of the present invention to define a simple, yet effective, method and apparatus for forming from a fiber slurry a fiber truss having relatively complex three-dimensional features that reinforce the fiber truss and reinforce composite structures containing the fiber truss. Fiber trusses formed according to the invention may be used individually or in combination to construct structural cores for sandwich panels having improved mechanical properties compared to corrugated and honeycomb panels. The fiber trusses may also take the form of various other molded structural fiber products, including egg cartons, produce trays, conformal packaging, molded packaging inserts, and molded pallets.
A preferred embodiment of the method includes two separate stages: a wet-forming stage, carried out at a wet-forming station, and a fiber truss drying and consolidating stage, carried out at a separate truss finishing station. In the wet-forming stage, a fiber slurry is added to a container bounded by smooth impervious straight walls. The bottom of the container is bounded by the forming surface of a rigid, foraminous wet-forming die that is fixed to the vertical walls of the container. This particular die will be referred to as the xe2x80x9cfixed wet-forming diexe2x80x9d, since it is typically fixed in relation to the container walls or it provides the reference point for relative motion of the various parts of the forming apparatus. In some embodiments, the fixed wet-forming die may move relative to the laboratory frame of reference or the die may move relative to the container.
The forming surface of the fixed wet-forming die produces the impression of the under-surface of the fiber truss. The container walls, which form a frame around the fixed wet-forming die, will be referred to as the xe2x80x9cdecklexe2x80x9d because of similarities with a device having the same name used in common paper-making. Use of a deckle eliminates the need for immersion of the device in a slurry, distinguishing the present invention from the prior art of Posch, et. al., which was discussed earlier.
A second rigid, foraminous wet-forming die that is moveable with respect to the fixed wet-forming die is used to form the upper-surface of the fiber truss. This die will be referred to as the xe2x80x9cmoveable wet-forming diexe2x80x9d. The forming surface of the moveable wet-forming die produces the upper-surface of the fiber truss. The forming surface of the moveable wet-forming die may be nested into the forming surface of the fixed wet-forming die. Nesting in this manner during truss formation allows production of thin fiber trusses having substantially uniform wall thickness throughout.
The moveable wet-forming die is backed by a plunger that pushes the moveable wet-forming die along the deckle walls toward the fixed wet-forming die. Force may be applied to the plunger using a hydraulic press or a screw press or the like. The piston-like assembly formed by the moveable wet forming die and the plunger will be referred to as the xe2x80x9cwet-forming punchxe2x80x9d because of an analogy with the punch used in more common molding operations.
Slurry is added to the interior of the deckle enclosure and fills a space above the fixed wet-forming die. The space occupied by the slurry within the deckle enclosure and above the fixed wet-forming die will be referred to as the xe2x80x9cslurry space.xe2x80x9d Fiber suspended in the slurry contained within the slurry space is rapidly concentrated near the forming surfaces of both the fixed wet-forming die and the moveable wet-forming die as the slurry is compressed between the dies and carrier fluid is discharged through fluid-discharge passages in both dies.
Carrier-fluid discharge and fiber deposition along the forming surfaces of both dies distinguishes the present invention from the inventions of Posch, et. al., Emery, and Setterholm and Hunt, all of whom teach fiber deposition on a single forming surface only. Discharge of carrier fluid and deposition of fiber on two forming surfaces, rather than a single forming surface, results in major improvements in forming speed and production of a smooth surface on both sides of the fiber truss. Because pressure is produced by applying force to a pair of wet-forming dies, the slurry may be discharged and the fiber compacted in one continuous motion of the moveable wet-forming die. There is no need to first complete the formation of a fiber layer, and then compact the fiber layer in a separate operation, as is sometimes done in conventional vacuum-forming and gaseous pressure-forming processes.
The carrier fluid is discharged from the slurry only through fluid-discharge passages in the pair of foraminous wet-forming dies. Carrier fluid is not discharged through the deckle encompassing the moveable wet-forming die, since the deckle is impervious in the present invention. The deckle of the present invention corresponds to the side walls which surround the moveable wall in the invention of Shetka. Unlike the present invention, the side walls in the invention of Shetka are pervious. Thereby, the side walls in the invention of Shetka provide a fluid-discharge path for the carrier fluid. The use of an impervious deckle and the elimination of carrier-fluid discharge through the deckle, disclosed herein, clearly distinguish the present invention from the prior art of Shetka.
If the deckle of the present invention were pervious, as taught by Shetka, slurry would be discharged from the edges of fiber masses. Edge discharge would have advantages if the fiber mass were very thick, since there would then be a large surface area for edge discharge. However, there would be no advantage in edge discharge from thin fiber trusses, to which the present invention applies, since the surface area available for fluid discharge from the edge of typical fiber trusses is very small. In fact, fluid-discharge openings in the deckle walls would lead to undesirable excess fiber deposition around the periphery of thin fiber trusses, as mentioned in the last section. Therefore, the present invention differs in both structure and function from the invention of Shetka.
In the present invention, wet-forming forces are applied by compressing the slurry between wet-forming dies using a hydraulic press or other press means, rather than by drawing the slurry through a single wet-forming die using vacuum suction or pushing the slurry through a single wet-forming die by pressurizing the air above the slurry. The disclosed wet-forming method that utilizes compressive forces between wet-forming dies clearly distinguishes the present invention from well-known conventional forming methods.
A sliding seal may be provided between the periphery of the moveable wet-forming die, or the plunger, and the interior walls of the deckle. With a good sliding seal, high slurry pressures may be attained within the deckle as the wet-forming dies press against the slurry, resulting in rapid discharge of carrier fluid through the foraminous dies and rapid formation of the fiber truss.
The sliding seal is preferably hermetic, although use of a close-fitting non-hermetic seal may also be advantageous. A sliding seal will decrease slurry flow substantially around the moveable wet-forming die, facilitating the generation of high slurry pressures and rapid formation speeds as the slurry is compacted between wet-forming dies. The smooth deckle walls provide an excellent sliding surface for the sliding seal, allowing formation of a tight seal against the deckle, and avoiding seal abrasion.
In the case of a sliding seal mounted to the wet-forming punch, composed of the moveable wet-forming die and the plunger, deckle fluid-discharge openings could produce severe wear and abrasion of the seal as the seal passed over the edges of the fluid-discharge openings on the perforated interior walls of the deckle. Therefore Shetka would not teach the use of a sliding seal against the deckle interior surfaces, further distinguishing the present invention from the prior art of Shetka. In fact, nowhere does Shetka suggest the use of a sliding seal in the practice or implementation of his invention.
A sliding seal could be mounted in the deckle walls of the present invention or the corresponding side walls of the invention of Shetka, although a seal is not suggested by Shetka. In this case, the seal would be fixed to the deckle and would slide against the exterior walls of the punch. As the punch squeezes out carrier fluid, the volume of the space between the punch and the deckle increases, drawing slurry into this space. Some fiber will then be deposited between the punch and the deckle, where it produces a frayed edge around the perimeter of the fiber truss. No such increase in volume or associated fiber deposition occurs if the sliding seal is fixed to the punch and slides along the deckle walls. Fixing the sliding seal to the punch and allowing the seal to slide along the deckle walls is therefore preferred to fixing the seal to the deckle walls and allowing the seal to slide along the punch.
Continuous pressure is applied to the wet-forming dies in the present invention until most of the carrier fluid is ejected and the fiber is compacted and molded into a fiber mass having the approximate shape of the finished fiber truss. The compacted and molded fiber mass will be referred to as a xe2x80x9cpre-form fiber truss.xe2x80x9d At the completion of the wet-forming stage, the pre-form fiber truss still contains a small quantity of carrier fluid that must be removed in order to promote inter-fiber bonding. Inter-fiber bonding solidifies the fiber truss and forms a finished product. With adequate compaction in the wet-forming stage, the pre-form fiber truss, though still wet and soft, has enough wet strength for relatively aggressive handling, due in large part to the three-dimensional reinforcement inherent in the structure.
Because the wet-forming dies in the present invention may be constructed of strong rigid materials, very high forming pressures may be applied to compress and shape the pre-form fiber truss in the wet-forming stage. High forming pressures produce pre-form fiber trusses having relatively little retained fluids. The low fluid content of highly compacted pre-form fiber trusses results in low energy costs for subsequent drying of the pre-form fiber trusses.
It has been discovered through experimentation that there are circumstances in which the fluid-discharge passages in the wet-forming dies do not need to thoroughly cover the forming surfaces of the dies. Rather, the fluid-discharge passages need only be placed along the recessed surfaces of the wet-forming dies. In these locations, the fluid-discharge passages produce a flow pattern that deposits fiber very effectively into channels or pockets in the wet-forming dies, facilitating fiber formation in these more inaccessible portions of the forming surfaces. Since the walls of the dies are thinnest in the recesses of the forming surface, small-diameter fluid-discharge passages in these recessed areas are easily drilled or otherwise placed in the wet-forming dies.
Force to compress the slurry and discharge the carrier fluid may be applied using a hydraulic press. With a standard hydraulic press, very high carrier fluid pressures can be developed to discharge the carrier fluid rapidly. In one set of experiments using a hydraulic press, almost all of the water in a dilute wood-fiber slurry could be discharged in less than 3 seconds using the disclosed method. This fluid-discharge rate represents a substantial improvement in the state of the art. The pre-form fiber trusses produced in these fluid-discharge tests had moisture contents of 60% using a forming pressure of approximately 200 psi.
The slurries used in the wet-forming stage of the invention may include a single fiber type or a variety of fibers types that are mixed with a single carrier fluid or various carrier fluid mixtures, the most common carrier fluid being water. Common fiber types that may be used with the present invention include fibers derived from wood, straw, grass, cane, reed, bast, seed hair, other lignocellulosic materials, and non-plant synthetic materials. Within the category of fibers derived from wood, fibers from re-cycled wood sources offer a particularly low-cost source of fiber. Use of re-cycled fiber also helps conserve our forest resources while reducing waste inventories. These recycled fiber sources include wood fibers derived from old corrugated containers (OCC), old newspapers, and mixed paper. Chemical additives, such as fiber bonding agents, impregnating resins, fire retardant, sizing agents or other wet-strength additives, preservatives, anti-bacterial agents, and insect repellants may be mixed with the fiber slurry or applied after the fiber has been dried to impart special physical properties needed in some applications.
In the second stage of the method, the moist pre-form fiber truss is removed from the wet-forming station and transferred to the finishing station, where the pre-form fiber truss is dried. During removal of the pre-form fiber truss from the wet-forming station, the exceptional wet strength of the pre-form fiber truss allows separation of the pre-form fiber truss from the forming surfaces of the wet-forming dies without tearing or rupturing the pre-form fiber truss. High levels of wet-strength, compared to other forming methods, also allow transfer of the compacted pre-form fiber truss at high acceleration rates, an important feature for high-speed mass-production of fiber trusses.
Drying at the finishing station is performed, preferably, by rapid compression of the pre-form fiber truss between a pair of heated finishing dies. To distinguish the heated finishing dies from their wet-forming counterparts, one of the dies will be referred to as the xe2x80x9cfirst hot-press diexe2x80x9d and its forming surface will be called the xe2x80x9cfirst heated forming surfacexe2x80x9d. The other finishing die will be referred to as the xe2x80x9csecond hot-press diexe2x80x9d and its forming surface will be called the xe2x80x9csecond heated forming surface.xe2x80x9d
Heat and pressure are maintained continuously between the hot-press dies until the fiber truss is dried to the desired level. With sufficiently rapid compression and heating of the pre-form fiber truss, a substantial amount of carrier fluid, typically liquid water, is driven out by the explosive expansion of steam that is created when the heated dies suddenly contact and rapidly compress the pre-form. Compared to relatively slow conventional press-drying, the rapid expulsion of liquid water in this process reduces the amount of water that must be vaporized. With less water to vaporize, less energy is required to dry the pre-form and the drying speed is increased. Drying speeds may be increased still further by applying a vacuum to one or both sides of the pre-form, as it is rapidly heated and compressed. These rapid-drying scenarios are commonly referred to in other contexts as xe2x80x9cimpulse drying.xe2x80x9d Impulse drying of thin paper sheets has been studied extensively for a number of years. Dramatic increases in drying speed and equally dramatic decreases in energy requirements have been reported for impulse drying in the paper industry. To our knowledge, impulse drying of more complex molded fiber products, including the fiber trusses of the present invention, has not been previously taught or implemented, distinguishing the present invention from all of the prior art.
Because the hot-press dies of the present invention can be made of metal, very high temperatures and pressures may be applied to the fiber truss as it is dried. For most practical purposes, temperatures of the hot press dies will be less than or approximately equal to 500 degrees F., which is dictated primarily by heat tolerance of the fiber rather than heat-tolerance of the dies. Consolidating pressures in excess of 2500 psi may be obtained with metal dies. The use of metal dies and the resulting ability to apply these high consolidating pressures and temperatures distinguishes the present invention from the invention of Setterholm, et. al.
The high-degree of fiber densification and strong inter-fiber bonding produced in the fiber truss when it is dried under continuous high-temperature and pressure, according to the present invention, creates a strong, dimensionally accurate, finished fiber truss having smooth surfaces on both of the extended sides of the truss. High temperature, high-pressure drying under continuous restraint also produces a finished fiber truss that is moisture resistant. Tolerance to moisture is confirmed by data reported by Gunderson in which moisture-resistant flat sheets of wood fiber were produced under similar conditions of continuous heat and pressure. Drying of fiber trusses may also be performed within the scope of the present invention without high-temperatures and without high-pressure restraint, resulting in a softer cushioning product, useful in many packaging applications.
If heated finishing dies are utilized in the drying stage of the present invention, the dies will most often be composed of metal. Metal dies may be actively and effectively heated with common heating sources such as steam, electric or gas heat. The forming surfaces of the finishing dies are in close contact with every surface of the fiber truss during truss consolidation and drying. For thin fiber trusses, thermal conduction paths are short, leading to very rapid and efficient heat transfer from actively heated metallic dies to all regions of the truss. If bonding agents are used, curing of the agents is also rapid because of the high thermal transfer rates. High thermal transfer rates, among other things, distinguish the present invention from the prior art of Setterholm, et. al.
In experimental tests of fiber truss formation performed according to the present method, drying times were 10 seconds for a truss wall thickness of 1.5 mm. and an overall structure thickness of 1 cm. This short drying time is considerably faster than drying times possible using the elastomeric mold elements disclosed by Setterholm et. al. Temperatures and compression rates in these tests were not optimal for rapid impulse drying. Nonetheless, some expulsion of liquid water, characteristic of impulse drying, was observed in the tests. More rapid drying rates, approaching those expected with impulse drying, would be obtained with increased temperatures and compression rates.
Typically, the forming surfaces of the heated finishing dies will closely match, or be identical to, the forming surfaces of the corresponding wet-forming dies. As such, the forming surfaces of the heated finishing dies nest into one another just as did the forming surfaces of the wet-forming dies. In order to produce the finished surface of the fiber truss, the finishing dies also substantially match the corresponding surfaces of the fiber truss. Because the finishing dies are capable of nesting into one another, a thin, light-weight finished fiber truss can be produced that has a substantially uniform wall thickness.
Once the fiber trusses are dried and consolidated at the truss finishing station, they may be joined together to form composite structural cores, or they may be used individually to form stand-alone structural cores. The cores may then be sandwiched between sheet liners, creating the panel-equivalent of an I-beam discussed earlier, and forming strong lightweight composite sandwich panels. The sheet liners may be composed of wood veneers, fiberboard, plastics, metals, or many other materials. Because the sheet liners are produced separately from the structural core, the physical properties of the core may be controlled independent of the physical properties of the sheet liners. Independent control of the sheet liner material relative to the core material enhances the versatility of these composite panels.
Since both the wet-forming dies and the finishing dies of the present invention are rigid, both sets of dies may be composed of a wide range of very durable materials, both metallic and non-metallic. Nearly any metal can be used to construct the dies including common metals such as aluminum and stainless steel. Ceramics may also be used, including the various grades of alumina, machinable glass-ceramics, and the less expensive machinable glass-mica composites. Plastics may be used as an inexpensive alternative to metals and ceramics in applications having sufficiently low forming pressures. Plastics that would be preferred in many applications include the various filled compounds which utilize base polymers of fluoroplastics, impregnated polyesters or polyethylene.
In contrast to prior methods for forming three-dimensional fiber objects using de-formable mold protrusions, taught by Setterholm, et. al., the protrusions extending from the forming dies of the present invention can be firmly and reliably attached to a base member using straightforward mechanical attachments, or through welding or brazing. In many cases, die protrusions may be machined directly into a solid blank, forming a very strong, durable one-piece forming die.
Because of the strength and durability of materials used in constructing the forming dies of the present invention, the dies will offer trouble-free performance in production applications and have a very long life expectancy compared to prior art molds made of relatively fragile elastomeric elements. In addition, the durable forming dies of the present invention will withstand the occasional presence of hard stray objects, such as staples, that might be accidentally contained in some slurries. These stray objects would destroy the elastomeric mold elements of the prior art. Tolerance to stray hard objects in slurries is a particularly important advantage when slurries are prepared from re-cycled sources of fiber in which material purity is often difficult to control.
The wet-forming method disclosed herein may be accomplished most simply by wet-forming individual fiber trusses one at a time in batch operations. In this case, all that is required is a single pair of wet-forming dies and a single deckle mounted to a single-opening press. Alternatively, several pre-form fiber trusses may be produced simultaneously using a series of die pairs and deckles mounted in a multi-opening press. It is conceivable that the fiber trusses could also be wet-formed continuously using moving molds in continuous belted presses, moving caul presses, counter-rotating roller presses, or the like.
The various fiber trusses that may be manufactured by the disclosed method have applications in a wide range of industries including packaging, material handling, construction, and furniture industries. A few of the specific products that can be fashioned using fiber trusses made according to the invention include pallets, bulk bins, heavy duty boxes, shipping containers, wall panels, roof panels, cement forms, partitions, poster displays, reels, desks, caskets, shelves, tables, and doors.